As scaleable, high power, continuous wave lasers have become available, their available wavelengths have tended to be in the near and mid infra-red regions of the electro-magnetic spectrum rather than in the visible and ultra violet regions where an increasing number of applications are emerging.
In order effectively shift the infra-red laser wavelengths into the visible and ultra violet regions under continuous wave conditions, it is necessary to pass the said infra-red laser beams at high intensities over relatively long lengths of non-linear optical media. This requirement eliminates the use of bulky non-linear crystals, which are effective under pulsed conditions, because the enormous laser beam intensities would heat up the crystal, initially causing thermally induced self-focussing effects and finally simply destroying the said bulk crystal.
It was known since the early 1960s that laser beams traversing the cores of multimode optical fibers gave rise to very high flux densities of many hundreds of watts per square centimeter. With the subsequent advent of single mode fibers, it was possible to inject a laser beam into a core only a few microns in diameter, that is, with a cross-sectional area of less than a millionth of a square centimeter so that less than a watt of continuous wave power was needed to achieve the enormous laser beam intensities exceeding one megawatt per centimeter square. Furthermore, with what is essentially a laser beam of merely one wavelength diameter traversing the core of the single mode fiber, it was not possible for self-focussing effects to occur and the heat dissipated per unit length of the fiber core was found to be negligible compared to the heating effects in bulk materials of similar thermal conductivity. It was eventually observed that, at the high laser beam intensities which could be maintained in single mode optical fiber cores, (normally a glass medium with linear optical properties), the fiber cores could become a non-linear frequency shifting medium for the transmitted laser beam. On the other hand, with fiber cores formed from non-linear optical material it became possible to convert over fifty percent of the propagating primary laser beam into its harmonics in the said fiber cores. However, once out of the fiber core, the laser beam expanded at angles of thirty degrees or more so that their intensities became very small as well as their total power which in general would be less than one tenth of a watt for continuous waves.
The present invention provides for the generation of scaleable power, frequency shifted laser beams which are phase-locked together to form what is essentially a single laser beam. In other words, if one inserts the invention into a given laser beam, then its frequency shifted component will be produced along the propagation path of the primary beam or at the appropriate angle to said primary beam if the relative phases of the primary beam are adjusted.
The invention can take the form of a bundle of single mode optical fibers with the appropriate non-linear cores or a solid block drilled with an array of holes into which non-linear fiber cores are inserted. Fluid cooling of the matrix medium can be used to maintain equal path lengths over the several centimeters of non-linear fiber cores required to achieve efficient conversion of continuous wave laser beam frequency. The invention can be positioned between two reflecting mirrors appropriately mirrored to generate standing waves within the said non-linear fiber cores. The invention can also be operated under pulsed as well as continuous wave conditions.